1. Field of the Invention
The invention pertains to the field of the reading of photosensitive cells, notably of the solid-state type, where each cell comprises two diodes mounted in series and mounted upside down with respect to each other, i.e. with opposite directions of conduction, at least one or these two diodes being a photodiode. A particular object of the method according to the invention is to increase the reading speed while, at the same time, preserving the precision and quality required in this reading, especially when the photosensitive cell has been exposed to a low intensity useful signal.
2. Description of the Prior Art
It is common practice to use, for example, photosensitive cells in solid state photosensitive devices wherein the photosensitive cells are placed in a matrix arrangement and form a photosensitive matrix. A photosensitive matrix has a system of conductors in rows and a system of conductors in columns. At each intersection between a row conductor and a column conductor, there is a photosensitive assembly or photosensitive cell hereinafter called a photosensitive dot. The photosensitive dots are thus organized in both rows and columns. Each photosensitive dot is connected between a row conductor and a column conductor. In fact, to each row conductor, there are connected as many photosensitive dots as there are columns of these dots and, to each column conductor, there are connected as may photosensitive dots as there are rows of these dots.
The number of photosensitive dots in a given surface determines the resolution of the image. There are known ways to make high capacity matrices of photosensitive elements, for example with 2000.times.2000 photosensitive dots, to obtain an image with dimensions of the order of 40 cm. by 40 cm. In this case, each photosensitive dot is located in an elementary surface zone, the maximum dimensions of which are 200 micrometers by 200 micrometers. An arrangement such as this enables the making of surface-type detectors which may be applied, notably, to radiology, provided that a scintillator is added, to the detection of particles (electrons, neutrons, etc.) and to reproduction graphics.
Each photosensitive dot has a photosensitive element such as a photodiode or phototransistor, sensitive to the visible or near visible light photons. These light photons are converted into electrical charges, and these electrical charges get collected in an electrical capacitor forming a storage capacity which may be formed by the capacitor of the photosensitive dot itself. A reading device can be used to interrogate the electrical status of the storage capacity and to convey the electrical charge, constituting the signal, towards a signal amplifier.
One of the main problems raised by the reading of photosensitive dots lies in an excessively great value of the electrical capacitance of the photosensitive cells. The effect of this capacitance is exerted particularly during the reading of the photosensitive dots, i.e. during the amplification of the photocharge developed by a photosensitive element, a photodiode for example, following its illumination. This capacitance of each photosensitive element is applied to the column conductors and to the reading amplifier to which this element is connected, and tends to cause deterioration in the signal-to-noise ratio.
It has to be further noted that it is desirable to avoid the application, to a column conductor and to the corresponding reading amplifier, of the capacitance of the photosensitive elements connected to row conductors other than the row conductor that is addressed at a given instant, i.e. it is desirable to achieve efficient uncoupling of the non-addressed photosensitive dots with respect to the column conductor. This means that each photosensitive cell must have an element acting as a switch kept in the open state, except during the stage for reading the photosensitive cell to which it belongs. With a view to simplifying the fabrication of solid state photosensitive matrices, a known way to fulfil the above-mentioned switch function is to use a diode series mounted with the photosensitive element.
An arrangement such as this is described in a French patent application No. 86 14058, publication No. 2 605 166, filed on 9th October 1986 on behalf of THOMSON-CSF. This patent application relates to a solid state photosensitive device, the method of its reading and the method of its fabrication. This patent application particularly describes a photosensitive device, which may be have a matrix or linear arrangement, wherein each photosensitive dot is formed by a photosensitive cell consisting of two diodes, mounted in series and upside down with respect to each other, i.e. with opposite directions of conduction. A first diode is connected to a row electrode, and the second diode is connected to a column electrode. In the example described in this patent application, the first diode has a capacitance which is about ten times smaller than the capacitance of the second diode. The result thereof is a reduction in the equivalent capacitance brought to a column conductor by each photosensitive cell connected to this column conductor. Furthermore, the reading method used is such that the first diode works like a switch which is put in the closed state solely during the phase for reading the cells connected to the row conductor to which it belongs. The result thereof is efficient uncoupling of the column conductors with respect to the photosensitive cells of the non-addressed lines.
However, a photosensitive cell, formed by an assembly of two diodes, mounted in series and upside down with respect to each other, the working of which is got by a standard reading method as described in the above mentioned document, has certain major drawbacks, especially the drawback of introducing a possibly considerable error in the reading of the charges generated at a photosensitive dot during the exposure of this dot to a light signal to be picked up. This fault is all the more pronounced as the signal is weak and as it is sought to obtain a high reading speed.
The method of the invention makes it possible to avoid these drawbacks, and is applicable to the reading of photosensitive dots or cells comprising two diodes, mounted in series and upside down with respect to each other as explained above.